Adding new node to existing SQL Server AlwaysOn

Hi,

Our AlwaysOn SQL Server 2012 already running with 2 Nodes on Windows Server 2012 R2 from past 1 year.

Now we are planning to add  NEW 3rd node. Does it requires any downtime for adding 3 Node to existing cluster? ]

Please suggest.

ENVIRONMENT: (VM Virtual Machines)

Node1      PrimaryManual                   --> Main Data Center

Node2     SecondaryManual     --> DR Location

Node 3     Primary          Automatic            --> Main Data Center